A cloudy day can really dim the promise of solar energy. But out in orbit, a collector could constantly harvest the sun’s photons. That's the idea being explored at the U.S. Naval Research Laboratory. And they have a prototype.

One side of the experimental module is covered in a photovoltaic panel that converts sunlight into electricity. In the middle sit electronic circuits that convert the electricity to radio waves. The other side has an antenna to beam the energy down to Earth, where it’s converted back to electricity.

The Navy module works well in a vacuum chamber that mimics the empty cold of space. In the test set-up, high-powered xenon lamps stand in for concentrated sunlight.

Of course, getting a working instrument into space is an expensive proposition. To cut costs, an orbiting solar array would probably have to be assembled in space, mostly likely by robots that don't even exist yet. And some people have qualms about concentrated radio frequency or microwave beams cutting down through the atmosphere to ground-based receivers.

But given the persistence of this idea—and the desperate need for clean energy—maybe space-based solar will have its day in the sun.

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